1. Field of the Invention
This invention is utilized for a printer, a display apparatus and the like, and relates to an image forming apparatus for forming a predetermined image by applying a voltage between a recording electrode and a recording medium and electrostatically applying a coloring material on the recording medium.
2. Description of the Prior Art
An example of such an image forming apparatus is disclosed in Japanese Patent Publication (Kokuku) No. 51-46707 (1976) (corresponding to U.S. Pat. No. 3,914,771). This apparatus carries a conductive magnetic toner 1, as a coloring material, on a nonmagnetic cylinder 3 by rotating a rotating magnet 2, and supplies the toner onto a recording electrode 4 made of a magnetic material, as shown in FIG. 7. A voltage corresponding to an image signal is then applied between a conductive layer 50b of a recording medium 50 (consisting of three layers comprising an insulating layer 50a, the conductive layer 50b, and a support layer (for example, a sheet of a mylar film) 50c in descending order from the surface) and the recording electrode 4. An image is formed by applying the toner 1 on the recording medium 50.
A display apparatus utilizing such an image forming apparatus is shown in FIGS. 8 and 9. That is, a predetermined image is formed on the recording medium 50 by adherence or nonadherence of the toner in accordance with the voltage applied by the recording electrode 4, and a recorded image observable by an eye E is formed on a display member at the right side in FIG. 8.
The belt-like recording medium 50 is formed by providing the conductive layer 50b and the insulating film 50a on the support layer 50c (consisting of a sheet, such as a mylar film or the like) in order to make the entire medium flexible. In order to reduce the cost of the recording medium 50 of the prior art, the conductive layer 50b is formed by depositing metal aluminum about 500 .ANG.-1000 .ANG. thick on a sheet 50c of mylar film in a vacuum. At one end portion of the belt-like recording medium, there is provided a region where the conductive layer 50b (made by vacuum deposition of metal aluminum) is exposed along the direction of rotation of the recording medium, and residual electric charges are removed by contacting a conductive brush 11 with the exposed conductive layer.
The above-described conventional apparatus, however, has the following disadvantages:
(1) A soft material made of carbon is used for the conductive brush 11 (FIG. 9) and contacts the exposed conductive layer of the recording medium. Although the material is soft, since the exposed conductive layer consists of a thin vacuum-deposited aluminum film (500-1000 .ANG.) as described above, the portion consisting of vacuum-deposited aluminum peels off by abrasion and becomes nonconducting due to prolonged friction with the conductive brush. As a result, electric charges within the recording medium generated while recording remain for a long period. The conventional apparatus has therefore the disadvantage that a previously recorded image is displayed again due to the residual electric charges. The apparatus also has the disadvantage that the potential of the conductive layer consisting of vacuum-deposited aluminum changes due to incomplete removal of the electric charges causing the coloring material (the conductive magnetic toner) to adhere to blank portions other than recorded to produce a so-called fog.
(2) Contact resistance between the conductive brush 11 made of carbon and the exposed aluminum-deposited conductive layer of the recording medium becomes, in some cases, not less than 100 K.OMEGA. during operation, while it is not more than 1 K.OMEGA. in a standstill state. When the contact resistance thus increases, a previously recorded image can not be erased due to the residual electric charges as described above. The apparatus also has the disadvantage that the toner adheres to blank portions to produce fog and the like.